The present invention relates to a metallic die for extruding a metal material, preferably a material comprised of Al or alloys thereof, and/or comprised of nonferrous metals, particularly Cu or alloys thereof, having an essentially flat, disc-shaped die body with at least one aperture, wherewith a second aperture segment adjoins (in the pressing direction) a first aperture segment, which first aperture segment corresponds to and gives the final form to the desired cross section of the extrusion, wherewith the second aperture segment is at least coincident with the first in its extent transversely to the pressing direction, and is preferably everywhere wider than said first aperture segment.
In general the extrusion procedure for metals is as follows: A billet or the like of the metal to be extruded is brought to its deformation temperature, and high press pressure is applied whereby one or more continuous metal extrusions are formed through the die opening or openings which gives the desired cross sectional shape to the extrusion. An extruding apparatus generally has a billet chamber with walls which resist heat and high pressure, in which the heated metal billet is inserted. The required extrusion pressure is then applied to the billet by means of, e.g. a press plunger which is slidable in the billet chamber. The end face of the billet chamber is closed off (except for the die aperture) by the die. The die may be held in position with respect to the lateral direction by means of a die holder, e.g. of a type which surrounds the die on the periphery thereof; and the die may be held in position with respect to the pressing direction by means of a pressure plate disposed in the tool holder. In the direction of the extruding movement, the die, which is exposed to high mechanical stresses, is supported on its side facing away from the billet chamber by means of a die support device, which device in turn is ordinarily supported against the aforementioned pressure plate of the tool holder.
For extrusion of nonferrous metals and their alloys, e.g. Al or Cu or alloys of these, dies are used which have die bodies which are essentially flat and disc-shaped, wherewith the shape of the forming aperture(s) is such that a first aperture segment, into which the metal being extruded first enters, has a cross sectional surface area and shape which essentially coincides with the final profile of the extrusion being produced. As one proceeds in the direction of metal movement, this final-forming aperture segment undergoes a transition to a generally wider segment, which second segment has at least the same cross sectional area as the first segment, but which generally is wider than the first in all directions transverse to the extrusion direction, which widening may be abrupt or gradual (in the nature of undercutting). The actual transition locus between the final forming first aperture segment and the subsequent, wider second aperture segment is generally non-smooth, i.e. it has a relatively sharp edge. In dies for extruding aluminum and copper and their alloys, there may not be an approximately conical inlet segment on the inlet side of the final-forming aperture segment, or such conical inlet segment may be very short.
The materials used for the described extrusion dies according to the state of the art, which materials are understood to be different depending on the nonferrous metal to be extruded, are comprised of specially treated round steel bar (hot-work steel) sawed into discs, which steel may be of the kinds designated as German steel Code Nos. 1.2343, 1.2344, or 1.2567, in a hardened and annealed, and possibly subsequently nitrided state. Such dies for Al are suitable for operating temperatures in the 450.degree. C. range, which temperatures occur at (heretofore) ordinary extrusion rates. For extrusion of Cu and its alloys, where the temperature of the heated billet is higher (750.degree.-1040.degree. C.), the dies ordinarily used are mounted dies of high alloy hot-work steel, e.g. of kinds designated as German steel Code Nos. 1.2581, 1.2678, 1.2886, or 1.2888, or of precipitation hardened Cu.sup.-, Mo.sup.-, or Ni steels, e.g. No. 1.6354, or of specialty alloys, particularly specialty alloys based on Co, Ni, or Mo.
In order to improve the economic efficiency of the extrusion process one must have available die materials which can withstand higher press speeds than formerly, which speeds lead to higher temperatures at the aperture and in the die. At the same time, it should be possible to employ dies with complicated aperture cross sectional shapes, and to increase economic efficiency by providing a plurality of apertures.
Temperatures which occur at the aperture are 600.degree.-640.degree. C., for example, when the high press forces are employed (in the range of 7000 metric tons which are required for these higher press speeds in the case of aluminum with a billet temperature of 450.degree. C. Under these conditions, the rest of the die body is heated to about 550.degree.-580.degree. C., and the tool holder is in the neighborhood of 200.degree. C. In order to be able to withstand the substantially increased stresses with regard to mechanical load, temperature, and frictional wear occasioned by the increased press pressure and speed, it was sought to fabricate the extrusion dies from metallic materials having elevated hot strength and resistance to frictional wear while hot, achieved by elevated levels of alloying components. However, this increasing of the alloy level has a detrimental effect on hot toughness, which is a serious drawback in view of the increased mechanical and thermal stressing and the condition that it be possible to produce profiles with complex shapes. When high alloy steels, e.g. high speed tool steels, have been used for dies for extruding Al and its alloys, or when Co.sup.-, Mo.sup.-, or Ni.sup.- based alloys have been used for dies for extruding Cu and its alloys, where (for the Al or the Cu extrusion) the apertures were of complex shape or of high cross sectional area or were several in number, it has been found that the usefulness of these hard die materials is limited because thermal and fire cracking occurs, so that it is practically inevitable that the tools will have to be reprocessed mechanically and thermally after a short service life. This situation is costly and engenders increased downtime. Moreover, the specialty alloys based on Co, Mo, or Ni, as used for dies for extruding bronze, brass, and copper, have a tendency to contract during use, whereby after a certain operating period they tend to loosen in their mounts.
The underlying problem of the present invention is to devise extrusion dies which do not have the described drawbacks despite the use of elevated press speeds, and which in particular achieve high extrusion speeds for the abovementioned metals and alloys, while having long service life, no appreciable risk of crack formation in the die even when extruding with die apertures of complex shape, and no appreciable contraction of the die.